This invention relates to a color scanner for plate making, and more particularly to improvements in a photoelectric scanning color separation machine for plate making. The machine involves scanning photoelectrically an original color picture to obtain picture signals, converting the picture signals into color separation signals by electronic circuits, controlling light exposure amounts for scanning a recording cylinder by the color separation image signals, and making color separation printing plates such as yellow, magenta, cyan and black.
In a color scanner of the cylinder scanning type, in general, scanning is performed by moving synchronously a pick-up head for picking up color signals from an original picture and an exposure head for recording a reproduction picture along the axes of, respectively, a picture cylinder with an original picture attached thereon and a recording cylinder with a recording film attached thereon, which cylinders are synchronously rotated.
In this case the pitch of the scanning lines, i.e., the distance the head moves per one revolution of the cylinder, is generally minute, such as 0.025 to 0.05 millimeters. Furthermore, slight irregularity of the pitch results in a conspicuous uneven stripe in the reproduction picture. Accordingly, it is necessary to provide quite an accurate driving mechanism.
In order to prevent shading, distortion, and so on, the pick-up head and the exposure head must be moved exactly parallel to the cylinder axis and be kept at an exact distance from the cylinders. Consequently, it is also necessary to provide quite an accurate moving mechanism.
Further, the cylinders are rotated at a fairly high speed of about 500 revolutions per minute. Thus, the machine must have a rigid construction to prevent ragged outlines in the reproduction picture being caused by vibration of the rotating parts of the machine.
Conventional color scanners for plate making have been constructed using an iron or aluminium base made by casting or welding, and flattening its upper surface by mechanical processing. Then mounted on said base by means of bearings is an original picture cylinder and a recording cylinder, and guide rails for supporting a pick-up head and an exposure head and a moving shaft for moving the heads are also disposed on the base.
However, in these previous scanners each cylinder and each head have been independently mounted on the base, and thus it has been quite difficult to assure accurate relative positions of the components, particularly parallel positions of the axes of the cylinders to the lines of motion of the heads. Hence, it has been necessary for expert engineers to perform delicate final adjustments of the finished machines. This has resulted in long assembly processes, and, accordingly, high cost.
Furthermore, machines constructed in the manner described above are liable to have their accuracy reduced by vibrations from outside, temperature variations and the like. Consequently, the machines are rather troublesome to maintain, and their durability is relatively short. This means poor economy.